1. Field of the Invention
This invention relates to saccharide compositions such as, for example, oligosaccharides, polysaccharides, glycolipids, and glycoproteins. More specifically, this invention relates to processes for preparing these and other saccharide compositions by enzymatic techniques.
2. Discussion of the Background
The term "carbohydrate" embraces a wide variety of chemical compounds having the general formula (CH.sub.2 O).sub.n, such as monosaccharides, disaccharides, oligosaccharides and polysaccharides. Oligosaccharides are chains composed of saccharide units, which are alternatively known as sugars. These saccharide units can be arranged in any order and the linkage between two saccharide units can occur in any of approximately ten different ways. As a result, the number of different possible stereoisomeric oligosaccharide chains is enormous.
Of all the biological polymer families, oligosaccharides and polysaccharides have been the least well studied, due in considerable part to the difficulty of sequencing and synthesizing their often complex sugar chains. Although the syntheses of oligonucleotides and polypeptides are well developed, there is currently no generally applicable synthetic technique for synthesizing oligosaccharides.
Numerous classical techniques for the synthesis of carbohydrates have been developed, but these techniques suffer the difficulty of requiring selective protection and deprotection. Organic synthesis of oligosaccharides is further hampered by the lability of many glycosidic bonds, difficulties in achieving regioselective sugar coupling, and generally low synthetic yields. These difficulties, together with the difficulties of isolating and purifying carbohydrates and of analyzing their structures, has made this area of chemistry a most demanding one.
Much research effort has been devoted to carbohydrates and molecules comprising carbohydrate fragments, such as glycolipids and glycoproteins. Research interest in such moieties has been largely due to the recognition that interactions between proteins and carbohydrates are involved in a wide array of biological recognition events, including fertilization, molecular targeting, intercellular recognition, and viral, bacterial, and fungal pathogenesis. It is now widely appreciated that the oligosaccharide portions of glycoproteins and glycolipids mediate recognition between cells and cells, between cells and ligands, between cells and the extracellular matrix, and between cells and pathogens.
These recognition phenomena can likely be inhibited by oligosaccharides having the same sugar sequence and stereochemistry found on the active portion of a glycoprotein or glycolipid involved in cell recognition. The oligosaccharides are believed to compete with the glycoproteins and glycolipids for binding sites on receptor proteins. For example, the disaccharide galactosyl .beta. 1-4 N-acetylglucosamine is believed to be one component of the glycoproteins which interact with receptors in the plasma membrane of liver cell. Thus, to the extent that they compete with potentially harmful moieties for cellular binding sites, oligosaccharides and other saccharide compositions have the potential to open new horizons in pharmacology, diagnosis, and therapeutics.
There has been relatively little effort to test oligosaccharides as therapeutic agents for human or animal diseases, however, as methods for the synthesis of oligosaccharides have been unavailable as noted above. Limited types of small oligosaccharides can be custom-synthesized by organic chemical methods, but the cost for such compounds is typically very high. In addition, it is very difficult to synthesize oligosaccharides stereospecifically and the addition of some sugars, such as sialic acid and fucose, has not been effectively accomplished because of the extreme lability of their bonds. Improved, generally applicable methods for oligosaccharide synthesis are desired for the production of large amounts of widely varying oligosaccharides for pharmacology and therapeutics.
For certain applications, enzymes have been targeted for use in organic synthesis as one alternative to more traditional techniques. For example, enzymes have been used as catalysts in organic synthesis; the value of synthetic enzymatic reactions in such areas as rate acceleration and stereoselectivity has been demonstrated. Additionally, techniques are now available for low cost production of some enzymes and for alteration of their properties.
The use of enzymes as catalysts for the synthesis of carbohydrates has been proposed, but to date enzyme-based techniques have not been found which are useful for the general synthesis of oligosaccharides and other complex carbohydrates in significant amounts. It has been recognized that a major limiting factor to the use of enzymes as catalysts in carbohydrate synthesis is the very limited current availability of the broad range of enzymes required to accomplish carbohydrate synthesis. See Toone et al, Tetrahedron Reports (1990) (45)17:5365-5422.
In mammalian systems, eight monosaccharides activated in the form of nucleoside mono- and diphosphate sugars provide the building blocks for most oligosaccharides: UDP-Glc, UDP-GlcUA, UDP-GlcNAc, UDP-Gal, UDP-GalNAc, GGP-Man, GDP-Fuc and CMP-NeuAc. These are the intermediates of the Leloir pathway. A much larger number of sugars (e.g., xylose, arabinose) and oligosaccharides are present in microorganisms and plants.
Two groups of enzymes are associated with the in vivo synthesis of oligosaccharides. The enzymes of the Leloir pathway is the largest group. These enzymes transfer sugars activated as sugar nucleoside phosphates to a growing oligosaccharide chain. Non-Leloir pathway enzymes transfer carbohydrate units activated as sugar phosphates, but not as sugar nucleoside phosphates.
Two strategies have been proposed for the enzyme-catalyzed in vitro synthesis of oligosaccharides. See Toone et al, supra. The first strategy proposes to use glycosyltransferases. The second proposes to use glycosidases or glycosyl hydrolases.
Glycosyltransferases catalyze the addition of activated sugars, in a stepwise fashion, to a protein or lipid or to the non-reducing end of a growing oligosaccharide. A very large number of glycosyltransferases appear to be necessary to synthesize carbohydrates. Each NDP-sugar residue requires a distinct class of glycosyltransferases and each of the more than one hundred glycosyltransferases identified to date appears to catalyze the formation of a unique glycidic linkage. To date, the exact details of the specificity of the glycosyltransferases are not known. It is not clear, for example, what sequence of carbohydrates is recognized by most of these enzymes.
Enzymes of the Leloir pathway have begun to find application to the synthesis of oligosaccharides. Two elements are required for the success of such an approach. The sugar nucleoside phosphate must be available at practical cost and the glycosyltransferase must be available. The first issue is resolved for most common NDP-sugars, including those important in mammalian biosynthesis. The problem in this technology however resides with the second issue. To date, only a very small number of glycosyltransferases are available. Access to these types of enzymes has been the single limiting factor to this type of carbohydrate synthesis.
It has been reported that most glycosyltransferases are difficult to isolate, particularly from mammalian source. This is because these proteins are present in low concentrations and are membrane-bound. Further, although a few glycosyltransferases have been immobilized, these enzymes have been reported to be unstable. To date only a very small number of glycosyltransferases are available from commercial sources, and these materials are expensive.
Much hope has therefore been put on future developments in genetic engineering (i.e., cloning) of enzymes, particularly since several glycosyltransferases have already been cloned, including galacto-, fucosyl-, and sialyltransferases. It is hoped that future advances in cloning techniques will speed the cloning of other glycosyltransferases and enhance their stability.
Accordingly, in light of their potential uses and the difficulty or impossibility to obtain them in sufficient quantities, there exists a long-felt need for general synthetic methods for the production of oligosaccharides, polysaccharides, glycoproteins, glycolipids, and similar species in an efficient, cost effective, stereospecific, and generally applicable manner.